Copending, coassigned U.S. patent application Ser. No. 09/573,815 filed May 17, 2000, “Photolithographically-patterned Out-of-Plane Coil Structures and Method of Making,” describes various photolithographically produced out-of-plane coil structures and methods of making such structures, which is incorporated herein by reference. These structures are particularly advantageous for forming several practical electrical components for integration on silicon substrates. Out-of-plane inductors, for example, offer several advantages over prior art planar inductors, in that out-of-plane structures minimize eddy currents induced in the underlying substrate and when out-of-plane coils are operated at high frequencies, skin and proximity effects are better controlled.
Out-of-plane coil structures place the coil axis parallel, rather than perpendicular, to the substrate plane. An out-of-plane coil structure includes a substrate and an elastic member having a first anchor portion fixed to the substrate, a loop winding and a second anchor portion connected to the substrate. The second anchor portion and the loop winding are initially fixed to the substrate, but are released from the substrate to become separated from the substrate. An intrinsic stress profile in the elastic member biases the second anchor portion away from the substrate forming the loop winding and causing the second anchor portion to contact the substrate. The resulting coil structure is out-of-the plane of the substrate. The loop winding may also include a plurality of turns.
Various techniques may be used to position the second anchor portion away from the takeoff point of the elastic member, either tangentially or axially. If the second anchor point is positioned tangentially from the takeoff point, the loop winding is generally in the shape of a distorted circle, i.e., the second anchor portion contacts the substrate in the same vertical plane as the first anchor portion. Various techniques may be used to position the second anchor portion tangentially from the takeoff point. For example, a mechanical stop can be fixed to the substrate at the desired location to catch the second anchor point while it is detached from the substrate. Also, the radius of curvature of the elastic member may be varied, such as by adding a load layer onto a portion of the elastic member or by patterning one or more openings or perforations into a portion of the elastic member.
Various techniques can be used to connect the second anchor portion to the substrate. For example, the second anchor portion can be soldered or plated to the substrate. Each anchor portion can be attached to a metal contact pad attached to the substrate, for providing electrical connectivity to other elements in a circuit. Preferably the elastic member is formed of a conductive material. Alternatively, a layer of a conductive metal, such as gold, copper or silver, may be formed on an inner surface, an outer surface, or both surfaces.
An alternate method for forming an out-of-plane coil based on closing half loop pairs of elastic members as disclosed in copending, coassigned U.S. patent application Ser. No. 09/591,262 filed Jun. 9, 2000, “Photolithographically-patterned Out-of-Plane Coil Structures and Method of Making” is incorporated herein by reference. Upon release the half loop pairs need only to be coarsely aligned to each other and connected together, such as by either plating or soldering. The loop halves need not be the same length. One side could be longer than the other to facilitate the overlap. A different release material may be used under each loop half to release the loop halves sequentially.
One difficulty in creating out-of-plane structures is ensuring that the elastic members used to form the loops are not bunched or entangled by hydrodynamic and surface tension forces when they are being released. It has been observed that aqueous release and drying of the released elastic members causes insufficiently stiff fingers to get pulled around by the air-liquid interface and stick together. The longer and narrower the released elastic members the greater is the problem. A related defect occurs when released elastic members intertwine. Another difficulty is providing enough contact area for the free end of the released elastic member where it makes mechanical contact for subsequent electroforming. A further difficulty is calibrating and maintaining the stress parameters in the metal deposition process in order to keep the diameter of the coil, and as such its inductance, within a few percent tolerance.